This is mostly a memory question - I'm having a hard time remembering this and I can't read the book right now.
On the first book, a snake at the zoo winks at Harry. For the sake of the question, we'll assume that snakes can in fact wink.
I have a fuzzy memory here - I think that it winked before Harry had any kind of interaction with the snake (was this before or after Harry broke the glass?)
So, why did the snake wink at Harry? Given that they had not met ever before.
I suppose that the only reason it would wink at Harry would be because it knows Harry can communicate with it. But this would imply that snakes are capable of sensing such ability on wizards - is there (other) evidence that this is true?